The present invention is directed to machine direction oriented high molecular weight, high density polyethylene (HMW HDPE) films having enhanced water vapor transmission properties and superior toughness properties, and articles made therefrom. The present invention is also directed to a method of making machine direction oriented HMW HDPE films having enhanced water vapor transmission properties and superior toughness properties, and articles made therefrom. The present invention is further directed to articles, particularly bags, made from machine direction oriented HMW HDPE films having enhanced water vapor transmission properties.
Low molecular weight high density polyethylene (LMW HDPE) film and medium molecular weight high density polyethylene (MMW HDPE) film are widely used in a number of commercial applications, including packaging materials, due to the desirable properties of the films. LMW HDPE and MMW HDPE films possess good moisture barrier properties, tensile strength, tear resistance, and compatibility with other polymeric materials for making composite films. Although these HDPE films have desirable properties for many packaging applications, there exists a need for better moisture barrier properties in some applications, such as for dry goods like cereal.
One method of improving the moisture barrier properties of HDPE film is to monoaxially orient the film. Several U.S. patents disclose a method of monoaxially orienting a HDPE film to improve the water vapor transmission of the film. See for example U.S. Pat. No. 4,183,893 to Eastwood et al. and U.S. Pat. No. 4,870,122 to Lu. Even though monoaxially orienting a HDPE film improves the water vapor transmission properties of the film, monoaxially orienting the film negatively impacts the puncture resistance and tear strength of the film, especially in the machine direction. Further, known methods of monoaxially orienting a HDPE film, where the goal is improved water vapor transmission rate, leave an undesirable high shrinkage rate when exposed to heat. In many packaging applications, the film material must be heat sealed to seal the packaged material from exposure to air and moisture. During the heat sealing process, nominally improved water vapor transmission rate film materials, using LMW and MMW HDPE materials, exhibit unsatisfactory shrinkage, puncture resistance, and tear strength, resulting in an unacceptable package.
Although LMW HDPE and MMW HDPE materials have experienced wide use as packaging materials, high molecular weight high density polyethylene (HMW HDPE) has been used very little as a moisture barrier packaging material due to its inherent processability problems. The low melt index of HMW HDPE leads to processing difficulties during an extrusion process. Further, the poorer, higher water vapor transmission rate of HMW HDPE material has prohibited many manufacturers from using HMW HDPE in packaging applications, due to the resulting need to produce much thicker films at too high a total material cost.
What is needed in the art is a film having superior water vapor transmission properties, minimal shrinkage when exposed to a heat sealing operation, high puncture resistance, and sufficient tear strength in both the machine and cross directions to be used as a packaging material for dry goods. Further, what is needed in the art is a film, which exhibits the desirable above-mentioned properties, and which may be downgauged to a desirable thickness that avoids the high costs associated with thicker films.
The present invention is directed to a unidirectionally oriented film comprising high molecular weight (HMW) high density polyethylene (HDPE). The oriented HMW HDPE films of the present invention possess substantially improved water vapor transmission rate (WVTR) and minimal shrinkage, while maintaining superior puncture resistance and tear strength.
The present invention is also directed to a method of making the above-described films. The method comprises forming a film comprising HMW HDPE, and subsequently orienting the film in the machine direction. The film may be formed by a film blowing process or cast extrusion process. The film may be formed as a single layer film or a multi-layer film comprising x layer sets, wherein each layer set comprises a layer containing HMW HDPE.
The present invention is further directed to articles of manufacture made from the above-described films. In one preferred embodiment, bags for packaging dry foods, such as cereal, are produced from the films.
These and other features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims.